This invention pertains to ethylene polymerization catalysts and more particularly to a method for preparing silica supported silyl chromate polymerization catalyts.
An ethylene polymerization catalyst system based on silyl chromate compounds is disclosed in U.S. Pat. No. 3,324,101. However, the catalysts disclosed in this reference afford ethylene polymers with melt indexes lower than those desired for special applications.
Chromium oxide supported titanium-containing catalyts for ethylene polymerizations are known but they require activation by heating in an oxidizing atmosphere to a temperature above 300.degree. C. and as high as 1000.degree. C. This activation is carried out after deposition of the chromium oxide on the support. See for example U.S. Pat. No. 3,622,521 and Netherlands patent application No. 72-10881.
In the present invention the titanium/aluminum modified silica support is heated at elevated temperatures prior to deposition of the silyl chromate.